The preparation of cyclic esters of alpha-hydroxycarboxylic acids such as lactide is an old and much studied process. These esters have generally been prepared by polymerizing the desired acid (lactic acid) to a relatively low molecular weight oligomer (oligomeric polylactic acid), then heating the oligomer, generally in the presence of a catalyst, to depolymerize it to the cyclic ester (lactide) which is recovered as a component of a vapor product stream.
Such a process suffers in that long reaction times and high temperatures are involved. The long residence times at high temperatures employed often results in side reactions, leading to unwanted isomers, charring of the polymer and yield loss.
An improvement over the processes of the prior art is found in U.S. Pat. No. 4,835,293, which discloses a depolymerization process for the production of lactide wherein "an inert gas, preferably nitrogen" (col. 3, line 33) is fed into a reactor so that the lactide vapors are carried out from the reaction zone along with the nitrogen. The resulting gaseous product stream is scrubbed with a polar organic solvent to recover the lactide.
In the process to manufacture the highly valuable, pure L-lactide, use of an alcoholic or aqueous solvent as a scrubber liquid for the recovery of the L-lactide from the vapor product is not entirely satisfactory. Lactide reacts in the alcoholic solution to form alkyl lactate, which not only constitutes a yield loss but further increases the solubility of all lactide in the scrubbing solution, further aggravating the yield loss problem. Also, in the manufacture of pure optical dimers such as L-lactide, the starting L-lactic acid used to make lactide always contains some D-lactic acid. Therefore, the lactide reaction product always contains some meso-isomer. Meso-lactide is more soluble in alcohol than L-lactide and concentrates in the alcohol. This, in turn, tends to increase the solubility of the L-isomer in the alcohol further increasing the yield loss. Thus, when the desired L-lactide, unreacted L-lactic acid and other lactic acid values are recovered from the alcoholic filtrate, which is normally recycled to the oligomer production portion of the process, they are accompanied by the meso lactide, which continues to build up in the system and eventually results in greater solubility losses of L-lactide and decreased efficiency of the process.
On the other hand, use of non-hydroxylic scrubbing solvents such as acetone, for example, which are non-reactive towards lactide and in which lactides are highly soluble, likewise presents difficulties inasmuch as such polar solvents solubilize the by-product hydroxycarboxylic acids causing them to be incorporated in the lactide product which would require further processing to separate the lactide from the acids.
Water as a scrubbing solvent is also unsatisfactory in that heat transfer to the water from the lactide is much faster than mass transfer; consequently, lactide precipitates as fog of particles which are difficult to capture in the absence of specialized and costly equipment.
Thus a need exists for a process of producing solvent-free lactide in the absence of a polar organic solvent in good yields without use of expensive equipment.
It is an object of this invention to provide a process that meets this need. It is a further object to provide an improved low temperature gas-assisted process, with low residence times and at high conversion rates for converting lactic acid to lactide via its oligomer at a high production rate and good yield using a fluorocarbon as a gas-stripping agent and as a quench solvent to remove lactide polymers, oligomers and dimers from a gas stream containing same.